Method for imparting hydrophilicity to substrate

ABSTRACT

The present invention relates to a method for imparting hydrophilicity to a substrate whereby high hydrophilic properties and water-holding properties can be maintained for a long period of time. According to the present invention, an SiO 2  film is formed directly or through an undercoat layer on a substrate under a reduced pressure of 100 Pa or less and immediately after the SiO 2  film is formed, the SiO 2  film is treated with water. Before forming the SiO 2  film, it is also desirable that an undercoat layer consisting of a TiO 2  film, Al 2 O 3  film, Nb 2 O 5  film, a laminated film prepared by laminating the TiO 2  film on the Al 2 O 3  film, a laminated film prepared by laminating the TiO 2  film on the Nb 2 O 5  film, or a low emissivity film be formed on a substrate and the SiO 2  film be then formed on the undercoat film to serve as an SiO 2  composite film.

TECHNICAL FIELD

The present invention relates to a method for imparting hydrophilicityto a substrate whereby high hydrophilic properties and water-holdingproperties can be maintained for a long period of time.

BACKGROUND TECHNIQUE

Various measures such as the use of defogging mirrors or eyeglasses havebeen taken to prevent fog due to vapor or rain. For example, there is amethod for applying a surface active agent to the surface of a glass orlens (hereinafter referred to as “substrate”), a method for applying awater absorbing agent to the surface of the substrate, a method forapplying a water repellent agent to the surface of the substrate, or amethod for coating the surface of the substrate with inorganichydrophilic substance by a wet method.

In the method for applying the surface active agent to the surface ofthe substrate, by positioning a hydrophilic group on the surface of thesubstrate, a water screen is uniformly leveled without water adhering toproduce a defogging effect.

The method for applying the water absorbing agent to the surface of thesubstrate is to produce a defogging effect by causing water adhering tothe surface of the material to be absorbed by the water absorbing agent.

In the method for applying the water repellent agent to the surface ofthe substrate, by providing a coating such as a fluoroethylene resinwhose surface tension is less on the substrate to increase the contactangle of water droplets, water adhering to the surface is repelled toproduce a defogging effect.

In the method for coating the surface of the substrate with an inorganichydrophilic substance, by providing a hydrophilic thin film with a smallcontact angle for the water droplets on the surface of the substrate,the water screen is made uniformly level without water adhering toproduce a defogging effect, wherein a wet method such as a sol-gelmethod or a spin coat method is adopted as a coating process.

DISCLOSURE OF THE INVENTION

In the method for applying the surface active agent, there is a problemthat the surface active agent cannot be used for a long time because itis easily swept away by water.

In the method for applying the water absorbing agent, there is a problemthat the water absorbing agent cannot withstand long term use because itwill soon be saturated if there is a lot of water or if an extremelythick water absorbing agent coating is required to allow the waterabsorbing agent to exhibit a water absorbing effect to some extent andas a result, the reflected image is distorted, or the water absorbingagent exhibits inferior flaw resistance because of the presence of anorganic material.

In the method for applying the water repellent agent, there is a problemthat though the water repellent agent repels water, it does not havesufficient defogging properties to repel even minute water droplet andit is difficult to maintain the defogging property for a long time.

Further, in the method for coating with an inorganic hydrophilicsubstance, there is a problem that uniform coating is not possible overa large area such as a mirror because it is difficult to control thecoating thickness in the order of nm. It is complicated because there isa plurality of treating processes such as spraying of coating solutionand burning, and it is difficult to maintain the defogging propertiesfor a long time because the mechanical strength on the coating surfaceis insufficient.

To solve the above-mentioned problems, according to the presentinvention, an SiO₂ film is formed directly or through an undercoat layeron a substrate under a reduced pressure of 100 Pa or less andimmediately after the SiO₂ film is formed, the SiO₂ film is treated withwater to selectively form silanol groups (SiOH) on the surface of theSiO₂ film. The silanol groups (SiOH) formed on the SiO₂ film exhibithydrophilic properties.

In the present invention, the term “SiO₂ film” is used to clearlyexpress a silicon dioxide film, but this also includes a film in whichthe ratio of an Si atom to an O atom is not always 1:2stoichiometrically. This also applies to an SnO₂ film.

A TiO₂ film, an Al₂O₃ film, a Nb₂O₅ film, a laminated film prepared bylaminating the TiO₂ film on the Nb₂O₅ film, or a low emissivity film isdesirable as the undercoat layer. In this case, it is desirable thatanother SiO2 film be formed between the undercoat layer and thesubstrate. It is also desirable that the SnO₂ film be selected as astructure for the low emissivity film. If the SnO₂ film which is formedthrough a laminated film prepared by laminating the SnO₂ film and theSiO₂ film on the substrate in turn by a CVD method is selected, it ispossible not only to decrease an interference reflected color of a thinfilm, but also to prevent seepage of alkalis (Na) from a glasssubstrate.

An SiN film is also effective as the undercoat layer provided to preventseepage of the alkalis (Na). A thickness of 1˜20 nm is desirable for theSiN film.

As described above, the SiO₂ film is formed under a reduced pressure of100 Pa or less. However, there is a method for forming the SiO₂ film bya plasma CVD under an atmosphere of 100˜1 Pa of which the maincomponents are silane gas and oxidizing gas, a method for forming theSiO₂ film by sputtering under a reduced pressure of 10˜0.1 Pa, or amethod for forming the SiO₂ film by vacuum deposition under a reducedpressure of 1 Pa or less, or the like. Above all, the method for formingthe SiO₂ film by sputtering is most excellent as a method for treating alarger area. When the SiO₂ film is formed by sputtering an Si targetunder an oxygen atmosphere, if 1˜400% (preferably 1˜200%, morepreferably 60˜80%) of argon or nitrogen is mixed relative to the oxygenin an oxygen atmospheric gas, it is possible to selectively form asilanol group by providing a large oxygen deficiency on the surface ofthe SiO₂ film.

In the case where a glass substrate is selected for the substrate, it isalso possible to obtain a hydrophilic treating surface by burning, afterthe water treatment to form silanol groups, the glass substrate at atemperature of 300˜500° C. under an atmospheric pressure in which 0˜400%of nitrogen is provided relative to air. In this manner, it is possibleto remove any organic substance adhering to the surface which wouldbecome water repellent component and to form a highly hydrophilic film.

It is also desirable that 0.1˜20% by weight of Al component be used todope an Si component in the SiO₂ film. In this manner, adsorption ofwater on the silanol group is stabilized and a film with highwater-holding properties can be formed.

Further, it is desirable that the substrate have transparency or mirrorreflection and the transparency or the mirror reflection be maintainedeven after the hydrophilic treatment.

PREFERRED EMBODIMENTS FOR WORKING THE INVENTION

Preferred embodiments of the present invention will now be explainedhereunder.

In a method for imparting hydrophilic properties to a substrateaccording to the present invention, an SiO₂ film is formed on thesubstrate by a film-forming method under a reduced pressure andimmediately after formation thereof, the SiO₂ film is treated withwater. There is no specific limit to the material for the substrate usedin the present invention as far as required to provide the substratewith a defogging function, hydrophilic function or water-holdingfunction. For example, a suitable material includes glass, a mirror, alens (including a plastic lens), tile or an aluminum panel.

As methods for forming an SiO₂ film on the substrate under a reducedpressure of 100 Pa or less, there are a plasma CVD method, a sputteringmethod, a vacuum deposition method and the like. The reason why asilanol group is formed by performing water treatment immediately afteran SiO₂ film is formed under a highly reduced pressure is because waterreacts before chemical adsorption of an organic substance onto thesurface of chemically unstable SiO₂ film formed by a film forming methodunder reduced pressure, oxidation at a surface-active site, orconnection of an organic substance at the active site is generated andtherefore the silanol group which contributes to the hydrophilicproperties is easily formed.

In particular, since the method for sputtering Si as a target in anoxygen atmosphere under a highly reduced pressure can form many activesites on the surface of the substrate, if this is combined with thewater treatment immediately after the formation of the film, it ispossible to form a large number of silanol groups (SiOH) with highlyhydrophilic properties on the surface of the SiO₂ film.

By forming a large number of silanol groups on the surface of the SiO₂film, the treated material can exhibit highly hydrophilic properties andwater-holding properties. Since the Si of the silanol group when formedby the sputtering method is firmly coated on the surface of thesubstrate, it is possible to maintain the hydrophilic properties andwater-holding properties for a long time.

Non-deionized water may be used in the water treatment. However, it isdesirable that distilled water, pure water, or acid solution adjusted byadding an inorganic acid such as hydrochloric acid, sulfuric acid,nitric acid or phosphoric acid, or an organic acid such as acetic acid,butyric acid or lactic acid to the pure water, or an alkaline solutionadjusted by adding a base such as sodium hydroxide, potassium hydroxideor ammonium hydroxide to the pure water be used. As a method for watertreatment, there is a method for dipping a material formed with an SiO₂film in water or a method for passing the material through water vapor,a method for spraying on the material, or the like.

As an atmospheric gas in the case where the SiO₂ film is formed, silanegas of 100˜1 Pa and an oxidizing gas or the oxidizing gas mixed withrare gas or hydrogen gas are used in the plasma CVD. In the vacuumdeposition method, there is a method for depositing an SiO₂ source byreducing the atmospheric pressure to 0.1 Pa or less, or a method forutilizing an SiOx (X=0˜2) source under an atmosphere of gaseous oxygenof 1˜0.1 Pa or the rare gas or mixed gas thereof.

An atmospheric gas used in the sputtering method may be gaseous oxygenalone, but a method for mixing 1˜400% by volume, preferably 1˜200% byvolume, more preferably 60˜80% by volume of argon gas and/or nitrogengas relative to oxygen may be used. When the mixed gas is used, an SiOxsuboxide film is formed and oxygen deficiency, i.e. a large number ofactive sites exist on the surface of the SiOx suboxide film. As aresult, it is possible to form a large number of silanol groups.

Further, as an other means for forming a large number of silanol groups,before forming the SiO₂ film, it is also desirable that an undercoatlayer consisting of a TiO₂ film, Al₂O₃ film, Nb₂O₅ film, a laminatedfilm prepared by laminating the TiO₂ film on the Al₂O₃ film, a laminatedfilm prepared by laminating the TiO₂ film on the Nb₂O₅ film, or a lowemissivity film be formed on a substrate and the SiO₂ film be thenformed on the undercoat film to serve as an SiO₂ composite film. Theundercoat layer has a function of imparting adhesion between the SiO₂film and the substrate, to improve durability of the film and to adjustthe surface irregularity. By treating the composite film with water, itis possible to selectively form the silanol group.

It is also desirable that Al be mixed with Si material in advance whenan SiO₂ film is formed or 0.1˜20% by weight, preferably 1˜10% by weightof an Al component be doped relative to an Si component of the SiO₂ filmby the treatment after formation of the silanol group by watertreatment. Since the Al component is doped, adsorption conditions ofwater to the silanol group are stabilized and as a result, thewater-holding properties of the SiO₂ film can be improved.

In this ease, it is desirable that another SiO2 film be formed betweenthe undercoat layer and the substrate. It is also desirable that an SnO₂film be selected as a low emissivity film. If the SnO₂ film formedthrough a laminated film prepared by laminating the SnO₂ film and SiO₂film on the substrate in turn by a CVD method is selected, it is alsopossible to decrease interference of reflected color of a thin film andto prevent seepage of alkali (Na) from a glass substrate.

It is also effective to form an SiN film of a thickness of 1˜20 nm asthe undercoat layer for the purpose of preventing seepage of alkali (Na)from the glass substrate.

In the case where the substrate is composed of glass material, it isdesirable that the substrate be burned at a temperature of 300˜500° C.after the water treatment to form silanol groups in an atmosphere inwhich 0˜100% of nitrogen is mixed relative to air. With this burning, itis possible to remove organic substances adhering to the surface whichbecomes a repellant component and to improve the hydrophilic properties.

The thickness of the SiO₂ film formed by the treating method of thepresent invention is adjusted to 1˜100 nm, preferably 1˜65 nm. Thereason for this is that in the case of under 1 nm, it is not possible tosufficiently exhibit the hydrophilic properties and even though thethickness is over 100 nm, it is not possible to expect an increasedeffect.

A film with a thickness of this range formed by the sputtering method isvery flat compared with the plasma CVD method or the vacuum depositionmethod, and the hydrophilic properties can be improved. The sputteringmethod also has a characteristic in that uniform hydrophilic film can beformed even on a substrate with a large area such as a mirror.

EMBODIMENTS AND COMPARATIVE EXAMPLES

The present invention will now be described with reference to specificembodiments and comparative examples.

Embodiment 1

A test substrate is prepared by cutting a float glass at 150 mm×150 mm.An undercoat layer of titanium oxide with a thickness of 500 nm isformed on the substrate by a sputtering method in an oxygen atmosphere.

Next, an SiO₂ film of a thickness of 10 nm is formed on the undercoatlayer by the sputtering method in the oxygen atmosphere and immediatelyafter that, it is dipped in the acid solution for water treatment toform a silanol group on the SiO₂ film.

The contact angle of a water droplet on a test panel prepared by theabove method was measured. As a result, the contact angle was 2° and hadhighly hydrophilic properties. This test panel was left in a room for 27days. The contact angle of the water droplet was measured again after 27days and, as a result, the same angle (2°) as before was found,confirming that highly hydrophilic properties could be maintained for along time. The results are shown in Table 1. A method for measuring thecontact angle of a water droplet is described below.

TABLE 1 Gas SiO₂ Water droplet composition film Water contact angle (°)No. Undercoat layer (O₂/Ar) thickness treatment 0 day 27th dayEmbodiment 1 TiO₂ (500 nm) 100/70 10 nm Acid water 2  2  2 Lowemissivity film 100/70 10 nm Acid water 2  9 (400 nm)  3 Low emissivityfilm 100/70 CVD10 nm Acid water 2 13 (400 nm)  4 Nil 100/70 15 nm Acidwater 2 20  5 Nil 100/70 65 nm Acid water 2 14  6 Nil — Wet method Acidwater 3 17 100 nm   7 Nb₂O₅ (10 nm) 100/70  7 nm Acid water 2 13 TiO₂(250 nm)  8 Nb₂O₅ (10 nm) 100/70 15 nm Acid water 2 15  9 SiN (10 nm)100/70 45 nm Acid water 2 15 10 SiO₂ (100 nm) 100/70  5 nm Acid water 213 Comparative Nil 100/0  15 nm Nil 2 32 example 1  2 Only lowemissivity — Nil Nil 2 34 film (400 nm)  3 Float glass of 3 mm — Nil Nil2 38  4 TiO₂ (500 nm) — Nil Nil 2 40 (Embodiments 2˜9 and Comparativeexamples 1˜4)

Test panels were prepared in the same manner as the embodiment 1 exceptthat the undercoat layer, gas composition, SiO₂ film thickness, andwater treatment conditions of the embodiment 1 were changed as shown inTable 1. These results are shown in Table 1.

Embodiment 10

A test substrate is prepared by cutting a float glass at 150 mm×150 mm.This substrate was dipped in a 30% solution of H₂SiF₆ for 90 minutes toform an SiO₂ undercoat film of a thickness of 100 nm.

Next, an SiO₂ film of a thickness of 5 nm was formed on the undercoatfilm by a sputtering method in an oxygen atmosphere and then dipped inan acid solution for water treatment, thereby forming a silanol group onthe SiO₂ film.

Embodiment 11

A test substrate was prepared by cutting a float glass at 150 mm×150 mm.An SiO₂ film of a thickness of 15 nm was formed on the substrate by asputtering method in a mixed atmosphere of oxygen and argon (Q₂/Ar % byvolume=100/70).

The SiO₂ film, once formed, is then dipped in an acid solution for watertreatment to form a silanol group on the SiO₂ film. Treatment conditionsare shown in Table 2.

TABLE 2 Undercoat Gas composition SiO₂ film Water No. Substrate kindlayer (O₂/Ar) thickness treatment Al dope Embodiment 11 Float glass of 3mm Nil 100/70 15 nm Yes Nil 12 Mirror of 5 mm Nil 100/70  7 nm Yes Nil13 Float glass of 3 mm Yes 100/70  7 nm Yes 8 wt % 14 Float glass of 5mm Nil 100/70  5 nm Yes 8 wt % 15 Float glass of 5 mm Nil 100/70  1 nmYes 8 wt % Comparative Float glass of 3 mm Nil 100/70 15 nm Nil 6 wt %example 5  6 Float glass of 5 mm Nil 100/40 15 nm Nil 6 wt %  7 Floatglass of 3 mm Nil 100/80 15 nm Nil 6 wt %  8 Float glass of 3 mm Nil100/60 15 nm Nil 6 wt %

The test panels prepared by the above-mentioned method were carefullymaintained in a room in an inclined and fixed condition. Water wassprayed on the panels on every date shown in Table 3 and the wettingdegree of the surface after 60 seconds was visually observed andevaluated. In the case where the surface is covered with dust andwettability is affected, the surface is wiped with a non-woven fabriccloth for further evaluation. Evaluation criteria are as follows:

⊚ More than 80% of the total area is wet. ◯ 50˜80% of the total area iswet. Δ 20˜40% of the total area is wet. X Wetting area is under 20% ofthe total area.

Evaluation results are shown in Table 3.

TABLE 3 Hydrophilic property evaluation No. 1^(st) day 3^(rd) day 6^(th)day 11^(th) day 16^(th) day 25^(th) day 29^(th) day 47^(th) day 51^(st)day Embodiment 11 ⊚ ⊚ ⊚ — ⊚ — ⊚ — ◯ 12 ⊚ ⊚ ⊚ — ⊚ — ⊚ — ⊚ 13 ⊚ — ⊚ ⊚ ⊚ —⊚ — ⊚ 14 ⊚ — ⊚ ⊚ ⊚ — ⊚ — ⊚ 15 ⊚ — ⊚ ⊚ ⊚ — ◯ — ◯ Comparative ◯ ◯ Δ — ◯ —Δ — Δ example 5  6 Δ X X Stopped  7 ◯ ◯ Δ — Δ — X — Δ  8 ◯ ◯ Δ — Δ — Δ —Δ (Embodiments 12˜15 and Comparative Examples 5˜8)

Each test panel was prepared and evaluated in the same manner as in theembodiment 11 except that the substrate kind, the undercoat layer, thegas composition, the SiO₂ film thickness, with or without watertreatment, and with or without Al dope were changed. Each condition isshown in Table 2 and evaluation results are shown in Table 3.

INDUSTRIAL APPLICABILITY

As described above, according to the present invention, immediatelyafter an SiO₂ film is formed on the substrate under a reduced pressureof 100 Pa or less, the SiO₂ film is treated with water to selectivelyform a silanol group (SiOH) on the surface of the SiO₂ film.Accordingly, it is possible to maintain highly hydrophilic propertiesand water-holding properties for a long time without affecting theoptical function and outer appearance of the substrate itself after adefogging treatment.

Accordingly, the substrate obtained by the method of the presentinvention can be expected to provide a fog prevention effect forimproving visibility in a high temperature and highly humid atmosphereand can be suitably applied to defogging mirror or window in a bath roomor the like. Also, since the SiO₂ film formed has an extremely smallcontact angle with water, it has the effect of removing any foulingcomposition contained in the water together with a water stream withoutretaining the fouling composition on the substrate. Accordingly, it ispossible to prevent fouling of the glass or panel.

Further, in the case where the glass substrate is used, the refractionfactor between the SiO₂ film formed and the substrate is almost thesame. Accordingly, there is no optical strain and transparency is alsohigh. In the case where the sputtering method is used as a method forforming the SiO₂ film, since control of the film thickness in the orderof several nm is possible, it is possible to obtain material with alarger area having a uniform SiO₂ film.

What is claimed is:
 1. A method for imparting to a substrate comprisingthe steps of forming an SiO₂ film directly or through an undercoat layeron the substrate under a reduced pressure of 100 Pa or less, treatingthe SiO₂ film with water immediately after formation of the film andforming silanol groups (SiOH) on the surface of the SiO₂ film.
 2. Themethod for imparting hydrophilicity to a substrate according to claim 1,wherein the undercoat layer is selected from the group consisting of aTiO₂ film, an Al₂O₃ film, a Nb₂O₅ film, a laminated film prepared bylaminating the TiO₂ film on the Al₂O₃ film, a laminated film prepared bylaminating the TiO₂ film on the Nb₂O₅ film, or a low emissivity film. 3.The method for imparting hydrophilicity to a substrate according toclaim 2, wherein the low emissivity film comprises a SnO₂ film.
 4. Themethod for imparting hydrophilicity to a substrate according to claim 1,wherein another SiO2 film is formed between the undercoat layer and thesubstrate.
 5. The method for imparting hydrophilicity to a substrateaccording to claim 3, wherein a laminated film comprising another SiO₂film on another SnO₂ film is formed between the SnO₂ film undercoatlayer and the substrate.
 6. The method for imparting hydrophilicity to asubstrate according to claim 1, wherein the undercoat layer comprises anSiN film.
 7. The method for imparting hydrophilicity to a substrateaccording to claim 6, wherein the thickness of the SiN film is 1˜20 nm.8. The method for imparting hydrophilicity to a substrate according toclaim 1, wherein the reduced pressure of 100 Pa or less is characterizedin that 1˜400% of argon or nitrogen is mixed relative to oxygen inoxygen atmospheric gas.
 9. The method for imparting hydrophilicity to asubstrate according to claim 1, wherein the thickness of the SiO₂ filmis performed by sputtering an Si target.
 10. The method for impartinghydrophilicity to a substrate according to claim 1, wherein thethickness of the SiO2 film is 1˜100 nm.
 11. The method for impartinghydrophilicity to a substrate according to claim 1, wherein thesubstrate is a glass substrate which is burned after the water treatmentat a temperature of 300˜500° C. under an atmosphere in which 0˜100%nitrogen is mixed relative to air.
 12. The method for impartinghydrophilicity to a substrate according to claim 1, wherein 0.1˜20% byweight of an Al component is doped relative to an Si component in theSiO₂ film.
 13. The method for imparting hydrophilicity to a substrateaccording to claim 1, wherein the substrate provides transparency or amirror reflection and the transparency or mirror reflection can still bemaintained after the hydrophilic treatment.